This invention relates to semiconductor devices and more specifically relates to novel vertical conduction superjunction type devices and their methods of manufacture.
Superjunction semiconductor devices are well known and generally provide plural layers of P and N regions connected between a source and drain region. In order to turn the device on in a forward conduction direction, current flow can proceed, for example, through the N type regions, which have a relatively high N type concentration. Thus, the device has a relatively low on resistance per unit area, or RDSON. To turn the device off, the adjacent P and N regions are caused to fully deplete, thus blocking current flow and turning the device off.
Superjunction devices of these types are shown in U.S. Pat. Nos. 5,216,275 and 4,754,310, and are also shown in copending application Ser. No. 60/113,641, filed Dec. 23, 1998 (IR-1676 Prov) in the name of Boden, and assigned to the assignee of the present invention.
The present invention provides a novel superjunction structure capable of blocking very high voltages, while having an ultra low on-resistance in the conduction mode.
In accordance with a first feature of the invention a plurality of deep P-type regions are shorted to the ground terminal placed within the N-type drift regions to assist in the depletion of these N-type regions during the blocking mode and to allow the use of even higher doping in the N-type regions. This further reduces the on-resistance contribution of the drift region, which is the principal source of on-resistance in devices in a high voltage range. The deep P-type regions are formed by etching deep trenches and doping the trench sidewalls with the appropriate P-type dose. The use of trench gates further allows increased density and reduced on-resistance.
In accordance with a second feature of the invention, the deep trenches are lined with an oxide film and then filled with a SIPOS (semi-insulating polysilicon) layer which is shorted to the drain through an opening in the oxide liner. The SIPOS is also shorted to the source at the top of the structure. This provides a highly resistive leakage path between source and drain causing the potential distribution to be uniform, thus reinforcing the RESURF effect of the trench sidewall doping.
In accordance with a third feature of the invention, the oxide used to fill the trench is replaced by alternate layers of oxide (SiO2) and nitride (Si3N4). The thermal coefficient of expansion of the nitride layer is greater than that of the oxide and of the parent silicon so that when the dielectric deposit cools, it shrinks as much as the silicon, reducing the material stress that would otherwise be present, had the dielectric had a different expansion coefficient from that of the silicon.